This invention relates to ferroelectric memory devices, and more particularly to multilayer electrodes for nonvolatile dynamic random access memory applications of ferroelectric materials.
Ferroelectrics have long been recognized as potential materials for nonvolatile storage of information. These materials are dielectric in nature and exhibit spontaneous polarization which can be reversed by application of a suitable electric field. The polarization P in these materials responds to an external electric field E in a hysteresis fashion and thereby the materials exhibit bistable properties (two distinct states of polarization) which remain even after removal of the electric field. It is this hysteresis feature that makes ferroelectrics suitable for nonvolatile memory storage. The dielectric nature of ferroelectrics and their ability to display bistable properties can be used to make a ferroelectric capacitor which stores binary digital information based on the polarization state of the material. This opens up the possibilities of integrating a ferroelectric capacitor into the existing Si and GaAs VLSI technology to make a commercial nonvolatile random access memory.
However, several problems need to be overcome before a commercially viable memory product is available. One of the foremost among these problems is the degradation properties of ferroelectric devices. Degradation properties include fatigue, low voltage breakdown, and aging. A common source for these degradation properties is the interaction between defects in the materials and the ferroelectric-electrode interface/grain boundaries in the ferroelectric capacitor.
Considering the problem of fatigue, ferroelectrics are noted to lose some of their polarization as the polarization is reversed. This is known as fatigue degradation, and is one of the prime obstacles to forming high quality ferroelectric films. Fatigue occurs because of defect entrapment at the ferroelectric-electrode interface. Asyrranetric electrode-ferroelectric interfaces and/or non-uniform domain distribution in the bulk can lead to asymmetric polarization on alternating polarity. This results in an internal field difference which can cause effective one-directional movement of defects like vacancies and mobile impurity ions. Since the electrode-ferroelectric interface is chemically unstable, it provides sites of lower potential energy to these defects relative to the bulk ferroelectric thereby causing defect entrapment at these interfaces (see Yoo et al, "Fatigue Modeling of Lead Zirconate Titanate Thin Films," Jour. Material Sci. and Engineering). This entrapment will result in a loss in polarization in the ferroelectric.
To overcome this problem caused by defects, it is necessary to control the defect concentration, defect migration to the interface, defect entrapment at the interface, and the state of the interface itself. Lattice mismatch, poor adhesion, and large work function differences between the electrode and the ferroelectric causes the interface to be chemically unstable. Therefore, it is necessary to choose an appropriate electrode which can reduce the lattice mismatch, work function differences, and the adhesion problem at the interface. The existing, commonly-used, metal electrodes such as Pt, Au, etc., do not satisfy these criteria because of the large differences in crystal structures between the electrode (metal) and the ferroelectric (ceramic), and because of the work function differences. To control the defect migration and entrapment it is necessary to reduce the abrupt compositional gradient between the electrode and the ferroelectric.